Cattle (Bos taurus) are intermediate hosts for four species of Sarcocystis, namely Sarcocystis cruzi, Sarcocystis hirsuta, Sarcocystis hominis, and Sarcocystis rommeli. Of these four species, mature sarcocysts of S. cruzi are thin-walled (<1 μm), whereas S. hirsuta, S. hominis, and S. rommeli have thick walls (4 μm or more). Here, we describe a new species of Sarcocystis with thin-walled sarcocysts in cattle. Two newborn calves were fed with sporocysts from the feces of a human volunteer who had ingested raw beef. The calves were killed 111 and 222 days later. In addition to thick-walled sarcocysts of S. hominis, both calves were coinfected with a Sarcocystis species that had a thin-walled sarcocysts, distinct from S. cruzi. The sarcocysts were mature, microscopic, up to 80 μm wide, and up to 1060 μm long. By light microscopy, the sarcocyst wall was thin (<1 μm thick) and had minute protrusions. By transmission electron microscopy, the sarcocyst wall had short, conical villar protrusions (vp) that were up to 0.5 μm long and up to 0.5 μm wide, similar to type 29. The vp on the sarcocyst wall lacked microtubules but had six or more disc-shaped plaques. The ground substance layer was smooth, approximately 0.5 μm thick, and without microtubules. The bradyzoites were 8-11 μm long. The structure of the sarcocyst wall was distinct from any species of Sarcocystis reported from livestock. This unique species is named in honor of Dr. Alfred Otto Heydorn who provided the sporocysts.